Easiest method is just do a linear scale based upon volumes. This will get you close, but not an exact replication of the original recipe. Differences in process can lead to significant differences in results.
The best method is to use software to scale the recipe to better fit your process. To make the most of these programs, you will need to define your process capabilities (efficiency, losses, boil off rate, etc.).
I use BeerSmith, but BrewFather and Brewer's Friend are also good choices. I believe all offer an initial trial period to test the software to see which works best for you. BeerSmith is computer based and the other two are on-line applications.